Mercury is a trace contaminant in all organic matter, including fossil fuels such as coal, petroleum and natural gas. Crude oil can contain a variety of heavy metal contaminants which, during the various processes which are utilized within an oil refinery, are distributed across many of the intermediate and product streams. Whilst the fate and effect of metals such as vanadium and nickel on refining processes are well understood, the concentrations and distribution of mercury are less clear although the presence of mercury in refinery products and discharges and emissions of mercury from the refinery are undesirable: for the fossil fuel and petrochemical industries, proper management of mercury can help prevent harmful effects on human and animal health, the environment, as well as on equipment.
Many sources of crude oil can contain mercury—crudes from Western Europe, Asia, the Middle East and North America have been reported to contain around 100 ppb of mercury. It is expected that much of the mercury in crude oil and refined product streams will exist in its elemental form. Elemental mercury is a relatively low boiling species and in this form it is likely to fractionate primarily into low boiling and naphtha streams as well as into light gas. This mercury can poison catalysts, prevent a final product stream from the refinery from meeting specification and also contribute to premature equipment failure. In the petrochemical industry, for example, the formation of mercury amalgams in the aluminum alloy cold boxes of ethylene crackers is a serious problem as described in Simultaneous Removal of Mercury and Water from Cracked Gas, Yan et al, Chemical Health & Safety, Nov,/Dec. 1995, 37. Various control and mitigation techniques have been proposed. U.S. Pat. No. 4,892,567 (Yan) describes the use of a silver zeolite-A catalyst for simultaneous mercury and water removal. Other proposals have involved the use of sulfur compounds to convert elemental mercury to insoluble sulfides which can then be removed by conventional filtration. U.S. Pat. No. 5,248,488 (Yan) discloses the use of various sulfur compounds for mercury removal followed by amine treatment to remove unreacted sulfur compounds. U.S. Pat. No. 4,786,483 (Audeh), for example, proposes control strategy by the use of peroxomonosulfates for removing both mercury and hydrogen sulfide. At the present time, Johnson Matthey Catalysts markets the PURASPECJM™ products which are designed to allow the effective removal of mercury from naphtha and other gaseous effluent streams.
The mercury removal method described by Yan in U.S. Pat. No. 4,892,567 using the silver-promoted molecular sieve (HgSIV) to absorb Hg in driers does raise the difficulty that the Hg/Ag amalgam decomposes during regeneration and the Hg in the off-gas must be managed in a secondary treater.
Another area of concern is the discharge of mercury in refinery waste streams, particularly waste water. Wastewater treating additives such as NALMET™ and METCLEAR™ from vendors such as Nalco and GE are reported to include sulfur-containing species such as dithiocarbamates that react with mercury to form a solid that can be removed from water by filtration. NUCON International offers a variety of chemicals for mercury removal under the title of MERSORB™ Mercury Adsorbents.
U.S. Pat. No. 5,667,694 (Cody), describes a heavy metal removal process using clay sorbents; U.S. Pat. No. 6,635,182 (Coleman) discloses the use of flocculants/scavengers such as the dithiocarbamates in the treatment of wastewater streams containing heavy metals including mercury by the formation of a floc which is subsequently removed by means of air flotation. U.S. Pat. No. 5,599,515 (Misra) discloses the use of dialkyldithiocarbamates to form stable mercury precipitates followed by flotation to remove the precipitate. U.S. Pat. No. 3,740,331 (Anderson) refers to the difficulties in removing ionic mercury by precipitation as a sulfide. US Patent Publication No. 2003/0082084 (Cort) discloses a two-step metal removal process applicable to mercury removal which combines hydroxide or sulfide precipitation with a physical removal. U.S. Pat. No. 6,165,366 (Sarangapani) discloses a process for mercury removal by oxidation using hypochlorite followed by filtration. U.S. Pat. No. 8,034,246 (Gustafsson) discloses a method for removing elemental and ionic mercury from wastewater streams by precipitation, flotation, filtration and carbon polishing.
A common way to manage mercury conventionally involved the use of what can be described as “on purpose sulfur addition”. For example, a refiner might add “Mercury Removal Units”, or MRUs, to the back end of the refinery, to remove mercury from specific product streams. These MRUs could consist of beds of a purchased, sulfur-impregnated solid over which the hydrocarbon products are passed. This would result in the formation of low-mercury products, plus a waste stream containing solid mercury sulfide. The waste stream, consisting of solid adsorbent with mercury sulfide, would then be disposed of properly.
Mercury removal units, sometimes called “Mercury Traps” may be added to the back end of a refinery, for example in the LPG train. These units often consist of a fixed bed containing a sulfur-impregnated solid such as the PURASPECJM™ absorbent mentioned above. The sulfur reacts with mercury to form solid mercury sulfide. The solid absorbent is dumped when it reaches its mercury capacity, and replaced with a fresh absorbent. The used solid absorbent from the MRU contains a higher concentration of mercury than the original crude oil, and can be disposed of in an environmentally acceptable manner. The hydrocarbon streams from the refinery, such as LPG, are now very low in mercury, and can be sold to customers without creating innumerable point sources of mercury pollution. This approach is conventionally adopted by refineries which knowingly process high mercury crudes.
Many refineries do not have mercury removal units. Instead, they avoid intentionally running high mercury crudes. However, to operate reliably in this mode with only low mercury crudes, a refiner requires a knowledge of the mercury content of each crude. This information is not readily available, for two reasons. First, it is difficult to measure the mercury content of a crude oil. The recent ASTM test D7622-10e1 Standard Test Method for Total Mercury in Crude Oil Using Combustion and Direct Cold Vapor Atomic Absorption Method with Zeeman Background Correction requires specialized equipment, as well as very special sample handling, in order to get an accurate result and, second, suppliers often blend crude oils together, either intentionally, when a supplier chooses to blend crude from one reservoir (possibly with higher mercury), with crude from another (possibly with low mercury) or unintentionally, when a supplier allows two crudes to be mingled, for example when filling a cargo ship carrying crude without having carefully cleaned the cargo tanks in the ship, which might contain some amount of the prior cargo.
With mercury management in the refining industry now becoming an area of increasing focus, various regulatory agencies and refinery customers are imposing limits on mercury in products and refinery discharges. Some refineries are considering and/or installing equipment to manage mercury. Other refineries reduce their mercury risk by avoiding high mercury crudes, some do not know the mercury content of their incoming crudes and do not have any active controls for mercury.